Conventionally, radio broadcasting and FM radio broadcasting in particular are a key broadcast medium that occupies an importance place worldwide. Especially in Europe and the United States, an extremely large number of broadcast stations provide broadcasting within a limited frequency band, and it is common practice to space two or more broadcast stations closely such as 100 kHz or 200 kHz apart.
Such spacing of broadcast stations inevitably causes interference from a broadcast wave of a nearby broadcast station, namely, adjacent channel interference. This being so, measures for detecting and removing adjacent channel interference are conventionally required of FM radio receivers. Many techniques have been proposed for adjacent channel interference detection methods and apparatuses that form the basis of such measures and are therefore considered to be particularly important.
For example, techniques of detecting adjacent channel interference with high accuracy by counting a frequency of an intermediate frequency signal (hereafter abbreviated as “IF signal”) using a counter and detecting a deviation from a predetermined frequency are disclosed in Patent References 1 and 2.
Moreover, a technique of detecting adjacent channel interference using a variation in DC component that is obtained as a result of smoothing an FM demodulated signal is widely known as described as a conventional technique in Patent References 1 and 2 and as a basic component in Patent Reference 3.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 11-239064    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 11-239065    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2000-312155